Impaired Stomatal Control Is Associated with Reduced Photosynthetic Physiology in Crop Species Grown at Elevated [CO2]

Physiological and Transcriptomic Dynamics in Mulberry: Insights into Species-Specific Responses to Midday Depression

Background/Objective: The midday depression of photosynthesis, a physiological phenomenon driven by environmental stress, impacts plant productivity. This study aims to elucidate the molecular and physiological responses underlying midday depression in two mulberry species, Ewu No. 1 (Ew1) and Husan No. 32 (H32), to better understand their species-specific stress adaptation mechanisms. Methods: RNA-seq analysis was conducted on leaf samples collected at three time points (10:00 a.m., 12:00 p.m., and 4:00 p.m.), identifying 22,630 differentially expressed genes (DEGs). A comparative Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis was performed to reveal the involvement of key metabolic and signaling pathways in stress responses. Results: Ew1 displayed enhanced stress tolerance by upregulating genes involved in energy management, water conservation, and photosynthetic processes, maintaining higher photosynthetic rates under midday stress. In contrast, H32 adopted a more conservative response, downregulating genes related to photosynthesis and metabolism, favoring survival at the expense of productivity. The KEGG analysis highlighted starch and sucrose metabolism and plant hormone signaling as critical pathways contributing to these species-specific responses. Conclusions: Ew1's adaptive molecular strategies make it more suitable for environments with variable light and temperature conditions, while H32's conservative approach may limit its productivity. These findings provide valuable insights for breeding programs aimed at improving stress tolerance and photosynthetic efficiency in mulberry and other crops, particularly under fluctuating environmental conditions.

Differential Drought Responses of Soybean Genotypes in Relation to Photosynthesis and Growth-Yield Attributes

Water scarcity leads to significant ecological challenges for global farming production. Sustainable agriculture depends on developing strategies to overcome the impacts of drought on important crops, including soybean. In this present study, seven promising soybean genotypes were evaluated for their drought tolerance potential by exposing them to water deficit conditions. The control group was maintained at 100% field capacity (FC), while the drought-treated group was maintained at 50% FC on a volume/weight basis. This treatment was applied at the second trifoliate leaf stage and continued until maturity. Our results demonstrated that water shortage exerted negative impacts on soybean phenotypic traits, physiological and biochemical mechanisms, and yield output in comparison with normal conditions. Our results showed that genotype G00001 exhibited the highest leaf area plant-1 (483.70 cm2), photosynthetic attributes like stomatal conductance (gs) (0.15 mol H2O m-2 s-1) and photosynthetic rate (Pn) (13.73 μmol CO2 m-2 s-1), and xylem exudation rate (0.25 g h-1) under drought conditions. The G00001 genotype showed greater leaf greenness by preserving photosynthetic pigments (total chlorophylls (Chls) and carotenoids; 4.23 and 7.34 mg g-1 FW, respectively) in response to drought conditions. Soybean plants accumulated high levels of stress indicators like proline and malondialdehyde when subjected to drought stress. However, genotype G00001 displayed lower levels of proline (4.49 μg g-1 FW) and malondialdehyde (3.70 μmol g-1 FW), indicating that this genotype suffered from less oxidative stress induced by drought stress compared to the other investigated soybean genotypes. Eventually, the G00001 genotype had a greater yield in terms of seeds pod-1 (SP) (1.90) and 100-seed weight (HSW) (14.60 g) under drought conditions. On the other hand, BD2333 exhibited the largest decrease in plant height (37.10%), pod number plant-1 (85.90%), SP (56.20%), HSW (54.20%), gs (90.50%), Pn (71.00%), transpiration rate (59.40%), relative water content (34.40%), Chl a (79.50%), total Chls (72.70%), and carotenoids (56.70%), along with the maximum increase in water saturation deficit (290.40%) and malondialdehyde content (280.30%) under drought compared to control conditions, indicating its higher sensitivity to drought stress. Our findings suggest that G00001 is a promising candidate to consider for field trials and further evaluation of its molecular signature may help breeding other elite cultivars to develop drought-tolerant, high-yielding soybean varieties.

Molecular Mechanisms of Plant Defense against Abiotic Stress

The climatic changes and anthropogenic factors in recent decades (global warming, drought, salinity, extreme temperature, environmental pollution) have led to an increase in the negative impact of environmental factors on plants. Abiotic stress strongly influences the important processes of plants and thus affects their growth and development. The effects of stressors on the plants depend on the intensity, frequency, and duration of stress, plant species as well as a combination of various stressors. Plants have developed different mechanisms to limit adverse environmental conditions. In the publications in this Special Issue, Molecular Mechanisms of Plant Defense against Abiotic Stress, new information on plant defense mechanisms against abiotic and biotic stress is presented. The studies help us better understand plants' protection mechanisms again global climate change.

On the Future Perspectives of Some Medicinal Plants within Lamiaceae Botanic Family Regarding Their Comprehensive Properties and Resistance against Biotic and Abiotic Stresses

Lamiaceae is one of the largest botanical families, encompassing over 6000 species that include a variety of aromatic and medicinal spices. The current study is focused on three plants within this botanical family: basil (Ocimum basilicum L.), thyme (Thymus vulgaris L.), and summer savory (Satureja hortensis L.). These three species contain primary and secondary metabolites such as phenolic and flavonoid compounds, fatty acids, antioxidants, and essential oils and have traditionally been used for flavoring, food preservation, and medicinal purposes. The goal of this study is to provide an overview of the nutraceutical, therapeutic, antioxidant, and antibacterial key features of these three aromatics to explore new breeding challenges and opportunities for varietal development. In this context, a literature search has been performed to describe the phytochemical profile of both primary and secondary metabolites and their pharmacological uses, as well as to further explore accession availability in the medicine industry and also to emphasize their bioactive roles in plant ecology and biotic and abiotic stress adaptability. The aim of this review is to explore future perspectives on the development of new, highly valuable basil, summer savory, and thyme cultivars. The findings of the current review emphasize the importance of identifying the key compounds and genes involved in stress resistance that can also provide valuable insights for further improvement of these important medicinal plants.

The functional significance of the stomatal size to density relationship: Interaction with atmospheric [CO2] and role in plant physiological behaviour

The limits for stomatal conductance are set by stomatal size (SS) and density (SD). An inverse relationship between SS and SD has been observed in fossil and living plants. This has led to hypotheses proposing that the ratio of SS to SD influences the diffusion pathway for CO₂ and degree of physiological stomatal control. However, conclusive evidence supportive of a functional role of the SS-SD relationship is not evident, and patterns in SS-SD may simply reflect geometric constraints in stomatal spacing over a leaf surface. We examine published and new data to investigate the potential functional significance of the relationship between SS and SD to atmospheric [CO₂] in multiple generation adaptive responses and short-term acclamatory adjustment of stomatal morphology. Consistent patterns in SS and SD were not evident in fossil and living plants adapted to high [CO₂] over many generations. However, evolutionary adaptation to [CO₂] strongly affected SS and SD responses to elevated [CO₂], with plants adapted to the 'low' [CO₂] of the past 10 million years (Myr) showing adjustment of SS-SD, while members of the same species adapted to 'high' [CO₂] showed no response. This may suggest that SS and SD responses to future [CO₂] will likely constrain the stimulatory effect of 'CO₂-fertilisation' on photosynthesis. Angiosperms generally possessed higher densities of smaller stomata that corresponded to a greater degree of physiological stomatal control consistent with selective pressures induced by declining [CO₂] over the past 90 Myr. Atmospheric [CO₂] has likely shaped stomatal size and density relationships alongside the interaction with stomatal physiological behaviour. The rate and predicted extent of future increases in [CO₂] will have profound impacts on the selective pressures shaping SS and SD. Understanding the trade-offs involved in SS-SD and the interaction with [CO₂], will be central to the development of more productive climate resilient crops.

Nitric oxide: A core signaling molecule under elevated GHGs (CO2, CH4, N2O, O3)-mediated abiotic stress in plants

Nitric oxide (NO), an ancient molecule with multiple roles in plants, has gained momentum and continues to govern plant biosciences-related research. NO, known to be involved in diverse physiological and biological processes, is a central molecule mediating cellular redox homeostasis under abiotic and biotic stresses. NO signaling interacts with various signaling networks to govern the adaptive response mechanism towards stress tolerance. Although diverging views question the role of plants in the current greenhouse gases (GHGs) budget, it is widely accepted that plants contribute, in one way or another, to the release of GHGs (carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O) and ozone (O3)) to the atmosphere, with CH4 and N2O being the most abundant, and occur simultaneously. Studies support that elevated concentrations of GHGs trigger similar signaling pathways to that observed in commonly studied abiotic stresses. In the process, NO plays a forefront role, in which the nitrogen metabolism is tightly related. Regardless of their beneficial roles in plants at a certain level of accumulation, high concentrations of CO2, CH4, and N2O-mediating stress in plants exacerbate the production of reactive oxygen (ROS) and nitrogen (RNS) species. This review assesses and discusses the current knowledge of NO signaling and its interaction with other signaling pathways, here focusing on the reported calcium (Ca2+) and hormonal signaling, under elevated GHGs along with the associated mechanisms underlying GHGs-induced stress in plants.

Pleiotropic effects of recombinant protease inhibitors in plants

Recombinant gene encoded protease inhibitors have been identified as some of the most effective antidigestive molecules to guard against proteolysis of essential proteins and plant attacking proteases from herbivorous pests and pathogenic microorganisms. Protease inhibitors (PIs) can be over expressed in transgenic plants to complement internal host defense systems, Bt toxins in genetically modified pest resistance and abiotic stress tolerance achieved through cystatins expression. Although the understanding of the role of proteolytic enzymes and their inhibitors encoded by both endogenous and transgenes expressed in crop plants has significantly advanced, their implication in biological systems still requires further elucidations. This paper, therefore, succinctly reviewed most recently published literature on recombinant proteases inhibitors (RPIs), focusing mainly on their unintended consequences in plants, other living organisms, and the environment. The review discusses major negative and unintended effects of RPIs involving the inhibitors' non-specificity on protease enzymes, non-target organisms and ubiquitous versatility in their mechanism of inhibition. The paper also discusses some direct and indirect effects of RPIs such as degradation by distinct classes of proteases, reduced functionality due to plant exposure to severe environmental stress and any other potential negative influences exerted on both the host plant as well as the environment. These pleiotropic effects must be decisively monitored to eliminate and prevent any potential adverse effects that transgenic plants carrying recombinant inhibitor genes may have on non-target organisms and biodiversity.

Physiological and Molecular Aspects of Two Thymus Species Differently Sensitive to Drought Stress

Drought is one of the most important threats to plants and agriculture. Here, the effects of four drought levels (90%, 55%, 40%, and 25% field capacity) on the relative water content (RWC), chlorophyll and carotenoids levels, and mRNA gene expression of metabolic enzymes in Thymus vulgaris (as sensitive to drought) and Thymus kotschyanus (as a drought-tolerant species) were evaluated. The physiological results showed that the treatment predominantly affected the RWC, chlorophyll, and carotenoids content. The gene expression analysis demonstrated that moderate and severe drought stress had greater effects on the expression of histone deacetylase-6 (HDA-6) and acetyl-CoA synthetase in both Thymus species. Pyruvate decarboxylase-1 (PDC-1) was upregulated in Thymus vulgaris at high drought levels. Finally, succinyl CoA ligase was not affected by drought stress in either species. Data confirmed water stress is able to alter the gene expression of specific enzymes. Furthermore, our results suggest that PDC-1 expression is independent from HDA-6 and the increased expression of ACS can be due to the activation of new pathways involved in carbohydrate production.

Ethanol Positively Modulates Photosynthetic Traits, Antioxidant Defense and Osmoprotectant Levels to Enhance Drought Acclimatization in Soybean

Drought is a major environmental threat to agricultural productivity and food security across the world. Therefore, addressing the detrimental effects of drought on vital crops like soybean has a significant impact on sustainable food production. Priming plants with organic compounds is now being considered as a promising technique for alleviating the negative effects of drought on plants. In the current study, we evaluated the protective functions of ethanol in enhancing soybean drought tolerance by examining the phenotype, growth attributes, and several physiological and biochemical mechanisms. Our results showed that foliar application of ethanol (20 mM) to drought-stressed soybean plants increased biomass, leaf area per trifoliate, gas exchange features, water-use-efficiency, photosynthetic pigment contents, and leaf relative water content, all of which contributed to the improved growth performance of soybean under drought circumstances. Drought stress, on the other hand, caused significant accumulation of reactive oxygen species (ROS), such as superoxide and hydrogen peroxide, and malondialdehyde, as well as an increase of electrolyte leakage in the leaves, underpinning the evidence of oxidative stress and membrane damage in soybean plants. By comparison, exogenous ethanol reduced the ROS-induced oxidative burden by boosting the activities of antioxidant enzymes, including peroxidase, catalase, glutathione S-transferase, and ascorbate peroxidase, and the content of total flavonoids in soybean leaves exposed to drought stress. Additionally, ethanol supplementation increased the contents of total soluble sugars and free amino acids in the leaves of drought-exposed plants, implying that ethanol likely employed these compounds for osmotic adjustment in soybean under water-shortage conditions. Together, our findings shed light on the ethanol-mediated protective mechanisms by which soybean plants coordinated different morphophysiological and biochemical responses in order to increase their drought tolerance.

Limited plasticity of anatomical and hydraulic traits in aspen trees under elevated CO2 and seasonal drought

The timing of abiotic stress elicitors on wood formation largely affects xylem traits that determine xylem efficiency and vulnerability. Nonetheless, seasonal variability of elevated CO2 (eCO2) effects on tree functioning under drought remains largely unknown. To address this knowledge gap, 1-year-old aspen (Populus tremula L.) trees were grown under ambient (±445 ppm) and elevated (±700 ppm) CO2 and exposed to an early (spring/summer 2019) or late (summer/autumn 2018) season drought event. Stomatal conductance and stem shrinkage were monitored in vivo as xylem water potential decreased. Additional trees were harvested for characterization of wood anatomical traits and to determine vulnerability and desorption curves via bench dehydration. The abundance of narrow vessels decreased under eCO2 only during the early season. At this time, xylem vulnerability to embolism formation and hydraulic capacitance during severe drought increased under eCO2. Contrastingly, stomatal closure was delayed during the late season, while hydraulic vulnerability and capacitance remained unaffected under eCO2. Independently of the CO2 treatment, elastic, and inelastic water pools depleted simultaneously after 50% of complete stomatal closure. Our results suggest that the effect of eCO2 on drought physiology and wood traits are small and variable during the growing season and question a sequential capacitive water release from elastic and inelastic pools as drought proceeds.

Elevated CO2 enhanced water use efficiency of wheat to progressive drought stress but not on maize

Global rising atmospheric CO₂ concentration ([CO₂]) and drought stress exert profound influences on crop growth and yield. The objective of the present study was to investigate the responses of leaf gas exchange and plant water use efficiency (WUE) of wheat (C3) and maize (C4) plants to progressive drought stress under ambient (a[CO₂], 400 ppm) and elevated (e[CO₂], 800 ppm) atmospheric CO₂ concentrations. The fraction of transpirable soil water (FTSW) was used to evaluate soil water status in the pots. Under non-drought stress, e[CO₂] increased the net photosynthetic rate (A_n) solely in wheat, and dry matter accumulation (DMA), whereas it decreased stomatal conductance (g _s) and water consumption (WC), resulting in enhanced WUE by 27.82% for maize and 49.86% for wheat. After onset of progressive soil drying, maize plants in e[CO₂] showed lower FTSW thresholds than wheat, at which e.g. g_s (0.31 vs 0.40) and leaf relative water content (0.21 vs 0.43) starts to decrease, indicating e[CO₂] conferred a greater drought resistance in maize. Under the combination of e[CO₂] and drought stress, enhanced WUE was solely found in wheat, which is mainly associated with increased DMA and unaffected WC. These varied responses of leaf gas exchange and WUE between the two species to combined drought and e[CO₂] suggest that specific water management strategies should be developed to optimize crop WUE for different species in a future drier and CO₂-enriched environment.

Exogenous Abscisic Acid Priming Modulates Water Relation Responses of Two Tomato Genotypes With Contrasting Endogenous Abscisic Acid Levels to Progressive Soil Drying Under Elevated CO2

Plants have evolved multiple strategies to survive and adapt when confronting the changing climate, including elevated CO₂ concentration (e[CO₂]) and intensified drought stress. To explore the role of abscisic acid (ABA) in modulating the response of plant water relation characteristics to progressive drought under ambient (a[CO₂], 400 ppm) and e[CO₂] (800 ppm) growth environments, two tomato (Solanum lycopersicum) genotypes, Ailsa Craig (AC) and its ABA-deficient mutant (flacca), were grown in pots, treated with or without exogenous ABA, and exposed to progressive soil drying until all plant available water in the pot was depleted. The results showed that exogenous ABA application improved leaf water potential, osmotic potential, and leaf turgor and increased leaf ABA concentrations ([ABA]_leaf) in AC and flacca. In both genotypes, exogenous ABA application decreased stomatal pore aperture and stomatal conductance (g _s), though these effects were less pronounced in e[CO₂]-grown AC and g _s of ABA-treated flacca was gradually increased until a soil water threshold after which g _s started to decline. In addition, ABA-treated flacca showed a partly restored stomatal drought response even when the accumulation of [ABA]_leaf was vanished, implying [ABA]_leaf might be not directly responsible for the decreased g _s. During soil drying, [ABA]_leaf remained higher in e[CO₂]-grown plants compared with those under a[CO₂], and a high xylem sap ABA concentration was also noticed in the ABA-treated flacca especially under e[CO₂], suggesting that e[CO₂] might exert an effect on ABA degradation and/or redistribution. Collectively, a fine-tune ABA homeostasis under combined e[CO₂] and drought stress allowed plants to optimize leaf gas exchange and plant water relations, yet more detailed research regarding ABA metabolism is still needed to fully explore the role of ABA in mediating plant physiological response to future drier and CO₂-enriched climate.

Facing metal stress by multiple strategies: morphophysiological responses of cardoon (Cynara cardunculus L.) grown in hydroponics

The contamination of environments by heavy metals has become an urgent issue causing undesirable accumulations and severe damages to agricultural crops, especially cadmium and lead which are among the most widespread and dangerous metal pollutants worldwide. The selection of proper species is a crucial step in many plant-based restoration approaches; therefore, the aim of the present work was to check for early morphophysiological responsive traits in three cultivars of Cynara cardunculus (Sardo, Siciliano, and Spagnolo), helping to select the best performing cultivar for phytoremediation. For all three tested cultivars, our results indicate that cardoon displays some morphophysiological traits to face Cd and Pb pollution, particularly at the root morphology level, element uptake ability, and photosynthetic pigment content. Other traits show instead a cultivar-specific behavior; in fact, stomata plasticity, photosynthetic pattern, and antioxidant power provide different responses, but only Spagnolo cv. achieves a successful strategy attaining a real resilience to metal stress. The capacity of Spagnolo plants to modify leaf structural and physiological traits under heavy metal contamination to maintain high photosynthetic efficiency should be considered an elective trait for its use in contaminated environments.

Contribution of PM2.5-Pb in atmospheric fallout to Pb accumulation in Chinese cabbage leaves via stomata

Foliar uptake of Pb is especially important when Chinese cabbage (Brassica rapa spp. pekinensis), having a large leaf surface area, is cultivated in North China during seasons with heavy haze. However, the mechanisms of foliar Pb uptake via stomata by Chinese cabbage exposed to atmospheric fallout are unclear. A field experiment was conducted to explore the impacts of Pb in particulate matter with sizes ≤ 2.5 µm (PM_2.5-Pb) from atmospheric fallout to Pb accumulation in cabbage leaves through stomata. Cabbage varieties with low-Pb-accumulation (LPA) and high-Pb-accumulation (HPA) were examined using inductively coupled plasma-mass spectrometry and scanning electron microscopy/energy-dispersive X-ray analysis. The ²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁷Pb ratios of PM_2.5, plants, and soil demonstrated that the major source of Pb in cabbage leaves was PM_2.5. The average width and length of the stomatal apertures were 7.14 and 15.61 µm for LPA cabbage and 8.10 and 16.64 µm for HPA cabbage, which are large enough for PM_2.5-Pb to enter the leaves. The HPA cabbage had significantly higher stomatal width-to-length ratios than the LPA cabbage, indicating that the former trapped much more PM_2.5-Pb and accumulated more Pb. These results clarify the contributions of the stomatal characteristics to PM_2.5-Pb accumulation in the edible parts of Chinese cabbage.

Elevated CO2 Modulates Plant Hydraulic Conductance Through Regulation of PIPs Under Progressive Soil Drying in Tomato Plants

Increasing atmospheric CO₂ concentrations accompanied by abiotic stresses challenge food production worldwide. Elevated CO₂ (e[CO₂]) affects plant water relations via multiple mechanisms involving abscisic acid (ABA). Here, two tomato (Solanum lycopersicum) genotypes, Ailsa Craig (AC) and its ABA-deficient mutant (flacca), were used to investigate the responses of plant hydraulic conductance to e[CO₂] and drought stress. Results showed that e[CO₂] decreased transpiration rate (E) increased plant water use efficiency only in AC, whereas it increased daily plant water consumption and osmotic adjustment in both genotypes. Compared to growth at ambient [CO₂], AC leaf and root hydraulic conductance (K _leaf and K _root) decreased at e[CO₂], which coincided with the transcriptional regulations of genes of plasma membrane intrinsic proteins (PIPs) and OPEN STOMATA 1 (OST1), and these effects were attenuated in flacca during soil drying. Severe drought stress could override the effects of e[CO₂] on plant water relation characteristics. In both genotypes, drought stress resulted in decreased E, K _leaf, and K _root accompanied by transcriptional responses of PIPs and OST1. However, under conditions combining e[CO₂] and drought, some PIPs were not responsive to drought in AC, indicating that e[CO₂] might disturb ABA-mediated drought responses. These results provide some new insights into mechanisms of plant hydraulic response to drought stress in a future CO₂-enriched environment.

Impact of Water Deficit on Seasonal and Diurnal Dynamics of European Beech Transpiration and Time-Lag Effect between Stand Transpiration and Environmental Drivers

In-situ measurements of tree sap flow enable the analysis of derived forest transpiration and also the water state of the entire ecosystem. The process of water transport (by sap flow) and transpiration through vegetation organisms are strongly influenced by the synergistic effect of numerous external factors, some of which are predicted to alter due to climate change. The study was carried out by in-situ monitoring sap flow and related environmental factors in the years 2014 and 2015 on a research plot in Bienska dolina (Slovakia). We evaluated the relationship between derived transpiration of the adult beech (Fagus sylvatica L.) forest stand, environmental conditions, and soil water deficit. Seasonal beech transpiration (from May to September) achieved 59% of potential evapotranspiration (PET) in 2014 and 46% in 2015. Our study confirmed that soil water deficit leads to a radical limitation of transpiration and fundamentally affects the relationship between transpiration and environmental drivers. The ratio of transpiration (E) against PET was significantly affected by a deficit of soil water and in dry September 2015 decreased to the value of 0.2. The maximum monthly value (0.8) of E/PET was recorded in August and September 2014. It was demonstrated that a time lag exists between the course of transpiration and environmental factors on a diurnal basis. An application of the time lags within the analysis increased the strength of the association between transpiration and the variables. However, the length of these time lags changed in conditions of soil drought (on average by 25 min). Transpiration is driven by energy income and connected evaporative demand, provided a sufficient amount of extractable soil water. A multiple regression model constructed from measured global radiation (RS), air temperature (AT), and air humidity (RH) explained 69% of the variability in beech stand transpiration (entire season), whereas (RS) was the primary driving force. The same factors that were shifted in time explained 73% of the transpiration variability. Cross-correlation analysis of data measured in time without water deficit demonstrated a tighter dependency of transpiration (E) on environmental drivers shifted in time (−60 min RS, +40 min RH and +20 min vapour pressure deficit against E). Due to an occurrence and duration of soil water stress, the dependence of transpiration on the environmental variables became weaker, and at the same time, the time lags were prolonged. Hence, the course of transpiration lagged behind the course of global radiation by 60 (R2 = 0.76) and 80 (R2 = 0.69) minutes in conditions without and with water deficit, respectively.

Wide vessels sustain marginal transpiration flux and do not optimize inefficient gas exchange activity under impaired hydraulic control and salinity

Plants optimize water use and carbon assimilation via transient regulation of stomata resistance and by limiting hydraulic conductivity in a long-term response of xylem anatomy. We postulated that without effective hydraulic regulation plants would permanently restrain water loss and photosynthetic productivity under salt stress conditions. We compared wild-type tomatoes to a transgenic type (TT) with impaired stomatal control. Gas exchange activity, biomass, starch content, leaf area and root traits, mineral composition and main stems xylem anatomy and hydraulic conductivity were analyzed in plants exposed to salinities of 1 and 4 dS m^-1 over 60 days. As the xylem cannot easily readjust to different environmental conditions, shifts in its anatomy and the permanent effect on plant hydraulic conductivity kept transpiration at lower levels under unstressed conditions and maintained it under salt-stress, while sustaining higher but inefficient assimilation rates, leading to starch accumulation and decreased plant biomass, leaf and root area and root length. Narrow conduits in unstressed TT plants were related to permanent restrain of hydraulic conductivity and plant transpiration. Under salinity, TT plants followed the atmospheric water demand, sustained similar transpiration rate from unstressed to salt-stressed conditions and possibly maintained hydraulic integrity, due to likely impaired hydraulic regulation, wider conduits and higher hydraulic conductivity. The accumulation of salts and starch in the TT plants was a strong evidence of salinity tolerance via osmotic regulation, also thought to help to maintain the assimilation rates and transpiration flux under salinity, although it was not translated into higher growth.

ABA-mediated modulation of elevated CO2 on stomatal response to drought

Elevated atmospheric CO₂ concentration (e[CO₂]) and soil water deficits have substantial effect on stomatal morphology and movement that regulate plant water relations and plant growth. e[CO₂] could alleviate the impact of drought stress, thus contributing to crop yield. Xylem-borne abscisic acid (ABA) plays a crucial role in regulating stomatal aperture serving as first line of defence against drought; whereas e[CO₂] may disrupt this fundamental drought adaptation mechanism by delaying the stomatal response to soil drying. We review the state-of-the-art knowledge on stomatal response to drought stress at e[CO₂] and discuss the role of ABA in mediating these responses.

Genetic controls of short- and long-term stomatal CO2 responses in Arabidopsis thaliana

BACKGROUND AND AIMS

The stomatal conductance (gs) of most plant species decreases in response to elevated atmospheric CO2 concentration. This response could have a significant impact on plant water use in a future climate. However, the regulation of the CO2-induced stomatal closure response is not fully understood. Moreover, the potential genetic links between short-term (within minutes to hours) and long-term (within weeks to months) responses of gs to increased atmospheric CO2 have not been explored.

METHODS

We used Arabidopsis thaliana recombinant inbred lines originating from accessions Col-0 (strong CO2 response) and C24 (weak CO2 response) to study short- and long-term controls of gs. Quantitative trait locus (QTL) mapping was used to identify loci controlling short- and long-term gs responses to elevated CO2, as well as other stomata-related traits.

KEY RESULTS

Short- and long-term stomatal responses to elevated CO2 were significantly correlated. Both short- and long-term responses were associated with a QTL at the end of chromosome 2. The location of this QTL was confirmed using near-isogenic lines and it was fine-mapped to a 410-kb region. The QTL did not correspond to any known gene involved in stomatal closure and had no effect on the responsiveness to abscisic acid. Additionally, we identified numerous other loci associated with stomatal regulation.

CONCLUSIONS

We identified and confirmed the effect of a strong QTL corresponding to a yet unknown regulator of stomatal closure in response to elevated CO2 concentration. The correlation between short- and long-term stomatal CO2 responses and the genetic link between these traits highlight the importance of understanding guard cell CO2 signalling to predict and manipulate plant water use in a world with increasing atmospheric CO2 concentration. This study demonstrates the power of using natural variation to unravel the genetic regulation of complex traits.

Carbon partitioning mechanisms in POTATO under drought stress

Potato (Solanum tuberosum) is an important food crop consumed all over the world, but it is generally sensitive to drought conditions. One of the major physiological processes affected by drought stress is carbon partitioning: the plant's choice of where to allocate its photoassimilates. Our aim was to investigate the molecular factors and possible bottlenecks affecting carbon partitioning during drought. We studied potato cultivars with contrasting drought responses in the greenhouse in the years 2013-2015, and further investigated the expression of genes involved in carbon partitioning and metabolite levels. Our results indicate that one of the most severe effects of drought stress on potato is the arrest of stolon differentiation and formation of tubers. We also identified some physiological traits like stomatal conductance and chlorophyll content as affecting carbon assimilation, partitioning and eventual tuber yield. The gene expressions and biochemical analyses highlight the various tissues prioritized by the plant for assimilate transport during drought stress, and give indications of what distinguishes drought tolerance and sensitivity of cultivated potato. Some of the key genes studied (like Sucrose synthase and Sucrose transporters) may be inclusive breeding targets for drought tolerance in potato.

Dust accumulation due to anthropogenic impact induces anatomical and photochemical changes in leaves of Centranthus ruber growing on the slope of the Vesuvius volcano

In Mediterranean ecosystems, some natural areas are exposed to severe anthropogenic impact. Especially in summer, the considerable number of tourists visiting such areas, often with vehicles, causes deposition of dust over the vegetation due to formation of powder clouds, also favoured by wind erosion, high temperature, low precipitation and incoherent soil structure. The main aim of this study was to analyse whether the deposition of dust can induce changes in leaf anatomical functional traits and in the efficiency of photosynthetic apparatus in Centranthus ruber, a species widespread in Mediterranean ecosystems. Leaf morpho-functional traits were quantified in plants growing at sites characterised by high (HD) and low (LD) dust deposition, in periods with high anthropogenic impact. Analyses included quantification of chlorophyll fluorescence emission parameters, photosynthetic pigment concentration as well as stomatal size and frequency, leaf lamina thickness, quantification of intercellular spaces and phenolics in the mesophyll through microscopy. The overall analysis suggested that the different conditions of dust deposition induced different adjustment of morpho-functional traits in leaves of C. ruber. High dust deposition shielded the leaf lamina, protecting the photosynthetic apparatus from excess light and favoured plant photochemical efficiency. Leaves exposed to low dust deposition showed higher accumulation of phenolic compounds, protecting chloroplast membranes and characterised by high thermal dissipation of excess light. Such adaptive phenomena can affect vegetation dynamics due to possible different species-specific plant responses, resulting in different plant competitiveness under the limiting conditions of Mediterranean environments.

Cold tolerance response mechanisms revealed through comparative analysis of gene and protein expression in multiple rice genotypes

Due to its tropical origin and adaptation, rice is significantly impacted by cold stress, and consequently sustains large losses in growth and productivity. Currently, rice is the second most consumed cereal in the world and production losses caused by extreme temperature events in the context of "major climatic changes" can have major impacts on the world economy. We report here an analysis of rice genotypes in response to low-temperature stress, studied through physiological gas-exchange parameters, biochemical changes in photosynthetic pigments and antioxidants, and at the level of gene and protein expression, towards an understanding and identification of multiple low-temperature tolerance mechanisms. The first effects of cold stress were observed on photosynthesis among all genotypes. However, the tropical japonica genotypes Secano do Brazil and Cypress had a greater reduction in gas exchange parameters like photosynthesis and water use efficiency in comparison to the temperate japonica Nipponbare and M202 genotypes. The analysis of biochemical profiles showed that despite the impacts of low temperature on tolerant plants, they quickly adjusted to maintain their cellular homeostasis by an accumulation of antioxidants and osmolytes like phenolic compounds and proline. The cold tolerant and sensitive genotypes showed a clear difference in gene expression at the transcript level for OsGH3-2, OsSRO1a, OsZFP245, and OsTPP1, as well as for expression at the protein level for LRR-RLKs, bHLH, GLYI, and LTP1 proteins. This study exemplifies the cold tolerant features of the temperate japonica Nipponbare and M202 genotypes, as observed through the analysis of physiological and biochemical responses and the associated changes in gene and protein expression patterns. The genes and proteins showing differential expression response are notable candidates towards understanding the biological pathways affected in rice and for engineering cold tolerance, to generate cultivars capable of maintaining growth, development, and reproduction under cold stress. We also propose that the mechanisms of action of the genes analyzed are associated with the tolerance response.

Altered stomatal dynamics induced by changes in irradiance and vapour-pressure deficit under drought: impacts on the whole-plant transpiration efficiency of poplar genotypes

Recent findings were able to show significant variability of stomatal dynamics between species, but not much is known about factors influencing stomatal dynamics and its consequences on biomass production, transpiration and water-use efficiency (WUE). We assessed the dynamics of stomatal conductance (g_s ) to a change of irradiance or vapour-pressure deficit (VPD) in two Populus euramericana and two Populus nigra genotypes grown under control and drought conditions. Our objectives were to determine the diversity of stomatal dynamics among poplar genotypes, and if soil water deficit can alter it. Physiological and morphological factors were investigated to find their potential links with stomatal morphology, WUE and its components at the whole-plant level. We found significant genotypic variability of g_s dynamics to both irradiance and VPD. Genotypes with faster stomatal dynamics were correlated with higher stomatal density and smaller stomata, and the implications of these correlations are discussed. Drought slowed g_s dynamics, depending on genotype and especially during stomatal closing. This finding is contrary to previous research on more drought-tolerant species. Independently of the treatment, faster stomatal dynamics were negatively correlated with daily whole-plant transpiration, presenting new evidence of a previously hypothesized contribution of stomatal dynamics to whole-plant water use.

Impact of Stomatal Density and Morphology on Water-Use Efficiency in a Changing World

Global warming and associated precipitation changes will negatively impact on many agricultural ecosystems. Major food production areas are expected to experience reduced water availability and increased frequency of drought over the coming decades. In affected areas, this is expected to reduce the production of important food crops including wheat, rice, and maize. The development of crop varieties able to sustain or improve yields with less water input is, therefore, a priority for crop research. Almost all water used for plant growth is lost to the atmosphere by transpiration through stomatal pores on the leaf epidermis. By altering stomatal pore apertures, plants are able to optimize their CO2 uptake for photosynthesis while minimizing water loss. Over longer periods, stomatal development may also be adjusted, with stomatal size and density being adapted to suit the prevailing conditions. Several approaches to improve drought tolerance and water-use efficiency through the modification of stomatal traits have been tested in the model plant Arabidopsis thaliana. However, there is surprisingly little known about the stomata of crop species. Here, we review the current understanding of how stomatal number and morphology are involved in regulating water-use efficiency. Moreover, we discuss the potential and limitations of manipulating stomatal development to increase drought tolerance and to reduce water loss in crops as the climate changes.

Effects of extreme changes in precipitation on the physiology of C4 grasses

Climatic patterns are expected to become more extreme, with changes in precipitation characterized by heavier rainfall and prolonged dry periods. Yet, most studies focus on persistent moderate changes in precipitation, limiting our understanding of how ecosystems will function in the future. We examined the effects of extreme changes in precipitation on leaf-level and ecosystem CO₂ and H₂O exchange of three native C4 bunchgrasses (Andropogon gerardii, Panicum virgatum, and Sorghastrum nutans) over 3 years. Grasses were grown in three precipitation treatments: extreme dry, mean, and extreme wet based on historical rainfall records. After 3 years, plants were 45% smaller in the extreme dry treatment relative to the mean and extreme high treatment, which did not differ. We also found that an extreme decrease in precipitation caused reductions of 55, 40, and 40% in leaf-level photosynthesis (A_net), stomatal conductance (g_s), and water use efficiency (WUE), respectively. Extreme increases in precipitation inhibited leaf-level WUE, with a 44% reduction relative to the mean treatment. At the ecosystem level, both an extreme increase and decrease in precipitation reduced net CO₂ and water fluxes relative to plants grown with mean levels of precipitation. Net water fluxes (ET) were reduced by an average of 74% in the extreme dry and extreme wet treatment relative to mean treatment; net carbon fluxes followed a similar trend, with average reductions of 68% (NEE) and 100% (R_e). Unlike moderate climate change, extreme increases in precipitation may be just as detrimental as extreme decreases in precipitation in shifting grassland physiology.

Differential coordination of stomatal conductance, mesophyll conductance, and leaf hydraulic conductance in response to changing light across species

Stomatal conductance (g_s ) and mesophyll conductance (g_m ) represent major constraints to photosynthetic rate (A), and these traits are expected to coordinate with leaf hydraulic conductance (K_leaf ) across species, under both steady-state and dynamic conditions. However, empirical information about their coordination is scarce. In this study, K_leaf , gas exchange, stomatal kinetics, and leaf anatomy in 10 species including ferns, gymnosperms, and angiosperms were investigated to elucidate the correlation of H₂ O and CO₂ diffusion inside leaves under varying light conditions. Gas exchange, K_leaf , and anatomical traits varied widely across species. Under light-saturated conditions, the A, g_s , g_m , and K_leaf were strongly correlated across species. However, the response patterns of A, g_s , g_m , and K_leaf to varying light intensities were highly species dependent. Moreover, stomatal opening upon light exposure of dark-adapted leaves in the studied ferns and gymnosperms was generally faster than in the angiosperms; however, stomatal closing in light-adapted leaves after darkening was faster in angiosperms. The present results show that there is a large variability in the coordination of leaf hydraulic and gas exchange parameters across terrestrial plant species, as well as in their responses to changing light.

Rising CO2 from historical concentrations enhances the physiological performance of Brassica napus seedlings under optimal water supply but not under reduced water availability

The productivity of many important crops is significantly threatened by water shortage, and the elevated atmospheric CO₂ can significantly interact with physiological processes and crop responses to drought. We examined the effects of three different CO₂ concentrations (historical ~300 ppm, ambient ~400 ppm and elevated ~700 ppm) on physiological traits of oilseed rape (Brassica napus L.) seedlings subjected to well-watered and reduced water availability. Our data show (1) that, as expected, increasing CO₂ level positively modulates leaf photosynthetic traits, leaf water-use efficiency and growth under non-stressed conditions, although a pronounced acclimation of photosynthesis to elevated CO₂ occurred; (2) that the predicted elevated CO₂ concentration does not reduce total evapotranspiration under drought when compared with present (400 ppm) and historical (300 ppm) concentrations because of a larger leaf area that does not buffer transpiration; and (3) that accordingly, the physiological traits analysed decreased similarly under stress for all CO₂ concentrations. Our data support the hypothesis that increasing CO₂ concentrations may not significantly counteract the negative effect of increasing drought intensity on Brassica napus performance.